The present invention relates to industrial control systems for synchronizing the operation of machines with a moving frame, for example, a conveyor belt or moving web, and in particular to an improved method and apparatus for such synchronization.
Industrial controllers are specialized computer systems used for the control of industrial processes or machinery, for example, in a factory environment. Generally, an industrial controller executes a stored control program that reads inputs from a variety of sensors associated with the controlled process and machine and, sensing the conditions of the process or machine and based on those inputs and a stored control program, calculates a set of outputs used to control actuators controlling the process or machine.
Industrial controllers differ from conventional computers in a number of ways. Physically, they are constructed to be substantially more robust against shock and damage and to better resist extreme environmental conditions than conventional computers. The processors and operating systems are optimized for real-time control and are programmed with languages designed to permit rapid development of control programs tailored to a constantly varying set of machine control or process control applications.
Generally, the controllers have a highly modular architecture, for example, that allows different numbers and types of input and output modules to be used to connect the controller to the process or machinery to be controlled. This modularity is facilitated through the use of special “control networks” suitable for highly reliable and available real-time communication. Such control networks (for example, ControlNet, EtherNet/IP) differ from standard communication networks (e.g. Ethernet) by guaranteeing maximum communication delays by pre-scheduling the communication capacity of the network, and/or providing redundant communication capabilities for high-availability.
As part of their enhanced modularity, industrial controllers may employ I/O modules dedicated to a particular type electrical signal and function, for example, detecting input AC or DC signals or controlling output AC or DC signals. Each of these I/O modules may have a connector system allowing them to be installed in different combinations in a housing or rack along with other selected I/O modules to match the demands of the particular application. Multiple or individual I/O modules may be located at convenient control points near the controlled process or machine to communicate with a central industrial controller via the special control network.
Often industrial control systems are applied to processes that includes a moving frame of reference such as: a conveyor belt, a web of printed material, a turntable, or the like. In such cases, the moving frame may be instrumented so that its position may be accurately determined, for example, with an encoder to provide for a motion signal providing a frame position relative to a predetermined reference location. This motion signal may be used to synchronize the acquisition of data related to the moving frame and/or the control of actuators that must operate relative to the moving frame.
In such control systems, the associated sensors and actuators (control devices) may be distributed in space at different locations with respect to the moving frame and thus the control of these devices must be corrected for their spatial offsets. This is normally done by means of offset variables defined in the control program, each offset variable representing the spatial offset of the control devices.
Particularly for high-speed processes, it may be necessary to adjust the position of the sensors or actuators with respect to the moving frame to allow sufficient time for control actions to be implemented. Such adjustments are complicated by the need to review the control program and identify the offset variables to make appropriate changes to these variable values.